Marlborough (often spelled Marlboro) is a city in Middlesex County, Massachusetts, United States. The population was 38,499 at the 2010 census. Marlborough became a prosperous industrial town in the 19th century and made the transition to high technology industry in the late 20th century after the construction of the Massachusetts Turnpike. Marlborough was declared a town in 1660. It later became a city because of the population size.

According to the United States Census Bureau, the city has a total area of 22.2 square miles (57 km2), of which, 21.1 square miles (55 km2) of it is land and 1.1 square miles (2.8 km2) of it (4.87%) is water. Marlborough is drained by the Assabet River. Within city limits are three large lakes, known as Lake Williams, Millham Reservoir and Fort Meadow Reservoir. A portion of Fort Meadow Reservoir extends into nearby Hudson. Marlborough is crossed by Interstate 495, U.S. Route 20 and Massachusetts Route 85. The eastern terimus of Interstate 290 is also in Marlborough.

AAs of the census of 2000, there were 36,255 people, 14,501 households, and 9,280 families residing in the city. The population density was 1,719.4 people per square mile (663.7/km≤). There were 14,903 housing units at an average density of 706.8 per square mile (272.8/km≤).